Handbook for Sound Engineers

876 Chapter 25

and the occasional genuine creative effect, the contrac-
tion EQ is well understood as both a noun and a verb.
This sonic mutilation uses frequency response curves
and shapes in degrees that have grown through an
uneasy mixture of operator needs and technical expedi-
ence/feasibility. One of today’s multiparametric console
channel EQs would have needed a rack full of tubes in
the fifties and sixties. Funny, they didn’t seem to need
such EQs then.
The delight (and maybe curse) of IC op-amp design
is that active filter (hence, EQ) implementation and
techniques have blown wide open, limited only by
economics, the largeness of the printed circuit board and
the smallness of the user’s fingers.
EQ curves can be roughly lumped into three user
categories: garbage disposal, trend, and area. High-pass,
low-pass, and notch filters that eliminate air-condi-
tioning burble, mic-stand rumble, breath noises, hum,
TV monitor line-frequency whistles, and excessive elec-
tronically generated noise are obviously in the business
of garbage disposal. Fig. 25-57A shows the sorts of
responses to be expected from these. Gentle hi-fi-type
treble and bass slopes and similar shelving curves estab-
lish response trends shown in Fig. 25-57B, while reso-
nance like, bell-shaped lift-and-cut filters manipulate
given areas of the overall spectral response, Fig.
25-57D. These are used to depress unwanted or irri-
tating aspects of a sound or, alternatively, to enhance
something at or around a given frequency that would
otherwise be lacking. As the curves differ, so do the
design techniques required.

25.11.1 Single-Order Networks

You can’t build a house until you have the bricks, so
they say. Fig. 25-58 has those bricks in the form of
combinations of basic passive components with a rough
guide to their input-output voltage transfer functions
(essentially the frequency responses). Assumptions are
that the Vin source impedance is zero and the Vo termi-
nation is infinite impedance.
Capacitative reactance decreases with increasing
frequency, working against the resistance to increas-
ingly short the output to ground with increasing
frequency in Fig. 25-58A, while in Fig. 25-58B the
capacitance steadily isolates the output from the input
with reducing frequency (rising reactance).
Inductors have entirely the opposite reactive charac-
teristics. Inductive reactance is directly proportional to
frequency, so the curves in Figs. 25-58C and D will be
of no surprise at all, being complementary to those
involving capacitance.

25.11.2 Single-Order Active Filters

Further useful curves are derived when the passive R, C,

and L elements are wrapped around an op-amp in the

classic inverting and noninverting amplifier modes, as

shown in Figs. 25-58E to L. All the curves in Fig. 25-58

are normalized to unity gain and the same center

Figure 25-57. EQ responses.

0.01 0.1 1 10 100

High

pass Notch

Low

pass

Frequency–kHz

A. High-pass, low-pass, and notch typical responses.

Lift

Cut

HF

(treble)

LF

(bass)

Frequency–kHz

0.01 0.1 1 10 100

Decibels

Frequency–kHz

C. Flat response.

B. Treble and bass shelving responses.

+20

+10

0

10

20

Frequency–kHz

Decibels

0.01 0.1 1 10 100

D. Bell or Gaussian responses.

+20

+10

0

10

20

Decibels

+10

0

10

20

Decibels

0

10

0.01 0.1 1 10 100

10